I. Field of the Invention
The present invention relates to a magnet for use in a magnetic resonance imaging device and to methods for making that magnet.
II. Background Information
Magnetic resonance imaging devices require that the target area to be imaged be subjected to a large uniform magnetic field on the order of 1.5 to 60 kiloGauss. In the past, electromagnets have been used which employ large conductive coils through which substantial amounts of current are passed. A magnetic field is thus created in the open space inside the coils and a return path is provided in the open space outside the coils. The magnetic field produced by such electromagnetic devices is not contained within any fixed return path and, therefore, such magnets have the disadvantage of being subjected to the adverse effects of nearby ferrous metallic objects which could result either in damage to those objects or to disruption of the uniform nature of the field inside the coils.
To avoid these disadvantages, prior art devices have been made available which employ permanent magnetic pole pieces which are separated from one another and between which the requisite magnetic field for magnetic resonance imaging is developed. In these devices, magnetic field conductive material, such as iron, is employed to provide a return path for the magnetic field between the poles. There are, however, several disadvantages to this type of prior art magnet. First, this type of magnet is extremely heavy and extremely difficult to manufacture and transport due to its weight and size. In addition, this type of prior art magnet typically has sharp corners in the return path which create discontinuities in the return magnetic field path. These discontinuities can adversely effect the uniformity of the field between the magnetic pole pieces, and can contribute to the leakage of field into the space outside the magnet. Furthermore, large, solid masses with appropriate magnetic field conductive properties are expensive to obtain.
In view of the foregoing, proprietary corporate research has been conducted on behalf of the assignee of the subject application. Although the results of these investigations were under the control of the assignee of the subject application at the time of the subject invention and, therefore, are not prior art, these investigations are nevertheless of interest in understanding the development of the subject invention.
Specifically, these investigations were directed toward the construction of a laminar magnet for use in a magnetic resonance imaging device. For example, as shown in FIG. 1, a magnet 10 was contemplated which comprised a plurality of stacked plates, for example illustrated plates 12a-12i. Plates 12a-12i could be stacked together to form magnet 10. Each of plates 12a-12i may comprise a top portion 14, a bottom portion 16, a first side portion 18, a second side portion 20, and oppositely facing teeth 22 and 24. Top portion 14 and bottom portion 16 are joined together at their edges by side portions 18 and 20 to form a generally square or rectangular shape. Extending down from top portion 14 toward bottom portion 16 is a top tooth 22 and extending upward from bottom portion 16 toward top portion 14 is lower tooth 24. As may be seen in FIG. 2, teeth 22 and 24 of plate 12a are narrower than teeth 22 and 24 of plate 12b, which, in turn, are narrower than teeth 22 and 24 of plate 12c.
Following prior art in the manufacture of electrical transformers, the grain orientation of the portions 14 through 24a of the plates 12a-12i is aligned insofar as possible parallel to the magnetic field. This grain orientation is shown in FIGS. 1 and 2 for plate 12a by the vertical arrows in portions 18, 20, 22, and 24 and by the horizontal arrows in portions 14, 15, 16, 17, 19, and 21.
Again following prior art in the manufacture of electrical transformers, the plates 12a-12i might be alternated with plates such as plate 12i of FIG. 2 whose grain orientation is similar to that in plate 12a except for the locations where the portions meet. Those locations in plate 12i are staggered oppositely to those in plate 12a, as shown in FIG. 2, thereby ensuring that the assembly will be mechanically strong at the locations where the portions meet. To this end, the side portions 18 and 20, which in plate 12a do not include the corners, are extended so as to include them in the case of side portions 18a and 20a in plate 12i. Likewise, the teeth 22 and 24, which in plate 12a do not extend to the outer edge of the plate, are so extended as shown by teeth 22a and 24a of plate 12i in FIG. 2. As a result, the top and bottom portions 14 and 16 of plate 12a are replaced by separate portions 15, 17, 19, and 21 of plate 12i.
Teeth 22a and 24a of plate 12i may also be made of progressively varying width.
FIG. 3 shows teeth 24 of plates 12a-12i when plates 12a-12i are assembled to form magnet 10. As may be seen in FIG. 3, the width of teeth 24 continues to get progressively larger from plate 12a to middle plate 12m, after which teeth 24 get progressively smaller so that the resultant structure of teeth 24, when plates 12a through 12i are assembled, is a general cylindrical pole piece as is illustrated in FIG. 1. Similarly, upper teeth 22 form a second generally cylindrical pole piece as is also shown in FIG. 1. Varying width teeth 24a of plates arranged with grain orientation like that of plate 12i may, of course, be used to selectively replace teeth 24 of plates arranged like plate 12a.
Although relatively easy to assemble and constructed of relatively inexpensive plates 12a-12i instead of a solid piece of iron, the magnet of FIGS. 1-3 has a fundamental disadvantage. Specifically, any magnetically conductive material has a preferred direction or orientation, as mentioned above, for conducting a magnetic field through that material. Even if portions 14 through 20 of each plate were made of independent sections of conductive material whose preferred orientation of magnetic conduction were aligned in the most preferable manner as shown by the arrows of plates 12a and 12i, there would nevertheless exist discontinuities at the points of connection between teeth 12 and top portion 14, top portion 14 and side portions 18 and 20, side portions 18 and 20 and bottom portion 16, and bottom portion 16 and teeth 24. Also, even with curved corners as shown in FIGS. 1 and 2, magnetic flux will emerge at these corners because the direction of magnetic conduction itself is not curved.
It is, accordingly, an object of the subject invention to provide a magnet for use in a magnetic resonance imaging device which is of economic laminar construction and within which a return path is formed without discontinuities in the preferred orientation of magnetic field conduction within that return path.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.